1. Field of the Invention
This invention relates to a method to enhance the efficacy of a fish vaccine and to a method to stimulate the immune system of a fish and to a composition and an agent useful in these methods.
2. Discussion of Background
Fish rearing in fish farms is increasingly evident. The advantages of aquaculture in comparison to fishing wild stocks is as apparent as the keeping of cattle compared with the hunting of wild animals. However, there are problems. The fish must be kept at a very high density and this means that the fish are susceptible to disease and, in particular, to the rapid spread of disease through the fish farm. Aquaculture techniques are improving all the time and the use of antibiotics, usually administered in the food, has been a major advantage in reducing disease. Nevertheless the use of antibiotics in this way can lead to the development of resistant strains of fish pathogens. There is also a popular sentiment against the use of antibiotics, growth hormones and the like in livestock and fish to be consumed by humans.
There has been some success in the use of vaccines, notably against diseases such as vibriosis and enteric redmouth. But there are still diseases for which vaccines are not effective or are of inadequate efficacy.
It is known in medicine that the efficacy of certain vaccines can be improved by the administration of compounds in addition to the vaccines.
It is known that certain glucans can be useful in curing human cancer by an apparent immunomodulating effect. Glucans are the anhydrides of glucose derivatives, for example cellulose, starch, dextrin and glycogen.
Relevant patents relating to glucans known to applicant include U.S. Pat. Nos. 4,098,661 issued Jul. 4, 1978, British Patent 1,061,043 dated Oct. 23, 1963 and Canadian Patent 968,286 issued May 27, 1975.
It has now been observed that certain glucans exhibit appreciable, advantageous effect in the use of vaccines for treating fish diseases. Of particular interest are .beta.-1,3-glucans and particularly the compounds schizophyllan (SPG) and scleroglucan (SG). These water soluble .beta.-1,3-glucans are reported to have triple helical structures. Schizophyllan is prepared by the precipitation in a culture filtrate of the fungi Schizophyllum commune, using acetone, ethanol or other water miscible solvent as a precipitant. Scleroglucan is obtained in a similar manner from a culture filtrate of a selected species of Sclerotium. The above compounds are represented by the following structural formulae: ##STR1##